Add support for date format to wxGridCellDateEditor too

This is needed to allow editing the cells using wxGridCellDateRenderer
with a custom format, otherwise the editor might parse the contents of
the cell differently from what is actually shown.

In particular, this ensures that using "date:%d.%m.%y" (or any other
custom format) as "cell type" works correctly, which wasn't the case
before.

include/wx/generic/grideditors.h

@@ -390,7 +390,9 @@ public:
 class WXDLLIMPEXP_ADV wxGridCellDateEditor : public wxGridCellEditor
 {
 public:
-    wxGridCellDateEditor() { }
+    explicit wxGridCellDateEditor(const wxString& format = wxString());
+
+    virtual void SetParameters(const wxString& params) wxOVERRIDE;
 
     virtual void Create(wxWindow* parent,
                         wxWindowID id,
@@ -414,6 +416,7 @@ protected:
 
 private:
     wxDateTime m_value;
+    wxString m_format;
 
     wxDECLARE_NO_COPY_CLASS(wxGridCellDateEditor);
 };